This invention relates to the measurement of Far End Crosstalk (FEXT) and determination of Equal Level Crosstalk (ELFEXT).
In twisted pair cabling as typically used for Local Area Network (LAN) systems, Attenuation, Near End Crosstalk, Return Loss and Equal Level Crosstalk (ELFEXT) transmission performance parameters are extremely important. For 1000BASE-T (1 Gbps Ethernet), which is currently under development by the IEEE 802.3ab committee, ELFEXT performance of a link is extremely important for satisfactory operation.
Referring to FIG. 1, a diagrammatic representation of a typical 1 Gbps Ethernet Link between a workstation and LAN equipment, the 1 Gbps Ethernet link between a workstation 10 and LAN equipment 12 uses 4 wire-pairs 14, 16, 18 and 20, with bi-directional transmission on each wire pair (transmit and receive). One of the signal transmission modes of 1 Gbps Ethernet on twisted pair cabling involves signals applied to one end of the link at the same time and traveling in parallel to the other end of the link. One major source of noise in this transmission mode results from coupling of one wire pair to another wire pair, as illustrated in FIG. 1. The impact of crosstalk on the wire pair 14 is shown at the top of FIG. 1 for a transmission from workstation 10 to LAN equipment 12. Crosstalk from the three other pairs 16, 18 and 20, couples into the top wire pair 14 as shown. At the receive input at the LAN equipment, this signal disturbs the desired signal, which is the attenuated signal from the workstation end. The signal-to-noise ratio from this contribution is therefore the (linear) ratio of crosstalk amplitude and amplitude of the attenuated signal. The crosstalk signal in this case is called "Far End Crosstalk" (FEXT). If both the FEXT and attenuation are expressed in dB, the signal-to-noise ratio expressed in dB is obtained by calculating the difference between the FEXT and attenuation. This ratio is called Equal Level Far End Crosstalk ("ELFEXT").
All wire pairs are noise sources for FEXT, and therefore add up. Since the signals on the wire pairs are generally uncorrelated, most often the combined effects of crosstalk from all wire pairs is summarized by taking the square root of the sum of the power of all crosstalk components (Power Sum FEXT, or Power Sum ELFEXT) to obtain an estimate for the total noise and signal-to-noise ratio at a receive input.
Other sources of noise in the 1 Gbps LAN system include Near End Crosstalk (NEXT) and Return Loss. NEXT performance is critical since signals arriving from the remote end of the link are disturbed by the output signals applied to the near end of the link. The bi-directional nature of signal on each wire pair results in reflected signals finding their way in the local receiver. Therefore, means are also designed in the 1 Gbps Ethernet system to compensate for this effect ("echo cancellation"). The 1 Gbps Ethernet system includes means for "learning" crosstalk performance and providing compensation for some of the disturbing effects. NEXT, ELFEXT and return loss are important link parameters and therefore must be measured accurately.
Referring now to FIG. 2, a graphical representation of a typical link, a local equipment jack 22 receives a patch cord plug 24 therein. The local equipment may comprise a workstation, for example, or in the case of a testing situation, may be a test instrument for measuring and testing network performance. Patch cord plug 24 defines one end of patch cord 26, the other end thereof suitably comprising another patch cord plug 28. Plug 28 connects to link jack 30, which may comprise a wall jack in a typical installation. Link jack 30 defines the connection to link cable 32 which extends to jack 34 of the link. There may be several connectors in link cable segment 32. At the last jack, a remote patch cord 36 includes plugs 38 and 40 and connects between jack 34 and the jack 42 at the remote equipment. The formal definition of the link excludes the connection to the equipment at the local and remote ends, and therefore is defined as being between point 44, which is just to the patch cord side of local patch cord plug 24, and point 46, which is just to the patch cord side of remote plug 40. The performance of a LAN system is measured at the link side of a mating connector, and therefore the performance measurement of the link should not include the impact from that connection. In Telecommunications Industry Association standard TSB-67, the standard cabling test configurations ("basic link" and "channel" ) specifically exclude this connection from the definition of the link. The international ISO/IEC 11801 cabling standard defines the channel configuration in the same manner. Moreover, when the transmission performance of a channel configuration is to be measured, the user patch cord (e.g., cord 26 or cord 36) is employed during the measurements. Since the standard plug on a user patch cord for a generic cabling system as defined in TIA/EIA-568-A or ISO/IEC 11801 and 1 Gbps Ethernet system is a modular 8-pin RJ-45 connector, the mating jack on the instrument has to be a modular 8-pin RJ-45 type. Unfortunately, the crosstalk performance of a modular 8-pin connector is relatively poor and has a material influence on the measured performance of a link with those connectors included in the result. The FEXT resulting from the connection to the measurement instrument system at the local and remote end must be compensated for in order to report accurate measurement values. The computed ELFEXT is subject to the same compensation.
For testing a basic link configuration, per TIA/EIA-568-A or the permanent link configuration per ISO/IEC 11801, a network technician may use a special patch cord, with the type of connector employed being one having low crosstalk characteristics. In such a case with a special patch cord, the transmission performance of the link using the special test cords is measured. However, by using such a special patch cord, the test configuration is not the actual configuration which ultimately carries the data during times other than in the test condition, since the user patch cord is removed during the test. Therefore, the measurements may not accurately represent the characteristics of the system once the special patch cord has been removed. Consequently, a method to accurately measure the channel configuration in addition to the basic link and permanent link test configuration is very desirable.